During January 1990 five cats from a recently imported colony of 48 cats at the NIHAC developed fatal respiratory tract infections. Bacterial cultures of lung tissues yielded CDC Group EF-4 from four cats. Although sporadic cases of feline pneumonia associated with this poorly defined bacterial group have been reported, this outbreak within a single colony appears to be unique. An unusually high incidence of fatalities, not due to EF-4 bacteria, continued to occur in this colony over the next few months. The purposes of this study are: (1) to further characterize the case- associated isolates by comparing them with control EF-4 strains obtained from other sources; (2) to assess the pathology of the lesions associated with EF-4 infection; and (3) to review the medical histories and clinicopathologic findings from cats which died of EF-4 pneumonia and from cats which died from other causes to determine if there were any underlying diseases which contributed to the fatalities in this colony. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS PAGE) analysis of the four outbreak-associated strains and eleven other epidemiologically unrelated EF-4 strains indicates that these strains may belong to a clonal group that is unique from the control strains. These results will be compared with clinical and pathologic findings to determine if the presence of this particular clone correlates with any particular pathogenic pattern. If these results are consistent, then virulence studies involving in-vitro and in-vivo systems will be designed.